The invention relates to a method and a configuration for determining a start of a first symbol and a carrier-frequency shift at a receiver end when a multicarrier signal containing a sequence of data symbols, which form a data block, is received once. The determination of the symbol start controls a symbol clock for demodulation of the individual symbols. The estimated value of the carrier-frequency shift is used as a manipulated variable for frequency correction in the receiver both during reception and during subsequent transmission of data signals. In this case, a test signal is transmitted from the transmission end at an uncertain time together with a data block, and is looked for and evaluated by a configuration at the receiver end. A design rule for test signals is specified, which allows the combined estimation of the start of the first data symbol and the carrier-frequency shift between the transmitter and receiver.
The invention is suitable for forward-acting digital synchronization of wire-free or wire-based receivers which process orthogonal frequency division multiplexing (OFDM) signals for transmission of individual data blocks, which are transmitted irregularly. The invention relates to the general case of single-shot synchronization, which can be carried out for each individual data block irrespective of preceding or future synchronization attempts. Its accuracy is suitable for high-rate OFDM signals, which may use higher-level modulation (for example 8-DPSK or 16-QAM) for high bandwidth efficiency. At present, OFDM is regarded as a suitable modulation technique for future broadband multimedia mobile radio systems and broadband wire-free networks.
Published, British Patent Application GB 2 307 155 A describes a synchronization method for OFDM, which uses guard intervals that are present in the signal.
The synchronization of OFDM signals has furthermore been dealt with, inter alia, in Published, European Patent Application No. 92113788.1, in a reference by F. Classen, titled xe2x80x9cSystemkomponenten fxc3xcr eine terrestrische digitale mobile Breitbandxc3xcbertragungxe2x80x9d [System Components For A Terrestrial Digital Mobile Broadband Transmission], in a dissertation at the RWTH Aachen, Shaker Verlag, Aachen 1996 and in conference publications by M. Schmidl, D. Cox, titled xe2x80x9cLow-Overhead, Low-Complexity [Burst] Synchronization For OFDMxe2x80x9d, Conference Proceedings, IEEE International Conference on Communications ""96, pages 1301-1306, and by M. Sandell, J. Beek, P. Bxc3x6rjesson, titled xe2x80x9cTiming And Frequency Synchronization In OFDM Systems Using The Cyclic Prefixxe2x80x9d, Conference Proceedings, International Symposium on Synchronization, Essen, Germany, December 1995, pages 16-19.
A number of previous works relating to the synchronization of OFDM receivers have proposed the transmission of a test signal of specific length at cyclic times, whose periodicity is evaluated by the receiver and is used to determine the start of a data block or of any carrier-frequency shift between the transmitter and receiver. Methods for this evaluation have been specified both for before and after the calculation of he fast fourier transformation (FFT) used for demodulation of FDM signals.
A disadvantage of the known methods and configurations is that they are each characterized by at least one of the below recited features. First, only a portion of the total synchronization of the receiver is dealt with, in which case the remaining synchronization tasks are presupposed to have been completed ideally; one example is the description of a method for estimation of the carrier-frequency shift, presupposing ideal symbol-clock synchronization. Second, regular repetition of test signals for receiver synchronization is stipulated, and/or averages over a plurality of synchronization sequences and test signals are required for adequate synchronization accuracy. While this approach is advantageous for broadcast radio applications, it is impossible, or feasible only with great complexity, for irregular transmission of data blocks in two transmission directions the computation operations to be carried out per synchronization sequence do not have the aim of minimum hardware processing complexity.
OFDM is a multicarrier modulation method. The transmitted OFDM signal s(t) in baseband includes a time sequence of individual OFDM symbol signals gi(t) of duration TS:                               s          ⁡                      (            t            )                          =                              ∑            i                    ⁢                                                    g                i                            ⁡                              (                                  t                  -                                      iT                    s                                                  )                                      ⁢                          xe2x80x83                        ⁢            where                                              (        1        )                                                      g            i                    ⁡                      (            t            )                          =                              ∑            k                    ⁢                                    S                              i                ,                k                                      ⁢                          ⅇ                              j2π                ⁢                                  xe2x80x83                                ⁢                                  kF                  Δ                                ⁢                t                                      ⁢                          b              ⁡                              (                t                )                                                                        xe2x80x83                                          b          ⁡                      (            t            )                          =                  {                                                                      1                  ,                                                            T                      G                                        ≤                    t                    ≤                    T                                                                                                                        0                  ,                  else                                                                                        xe2x80x83            
The summation index i represents the symbol clock, and k represents the subcarrier of the frequency kFxcex94. The OFDM symbol signal gi(t) contains the superposition of M (for example M=49) subcarriers ej2xcfx80Fxcex94t which are modulated independently of one another by the complex data symbols Si,k. The vector of all the symbols Si,k for a fixed symbol clock value i is referred to as the symbol block si. The superposition, also called modulation, is carried out digitally by an inverse fast fourier transformation (IFFT) of length NFFT. NFFT greater than M where M input values of the IFFT are identical to Si,k, and the remaining (NFFTxe2x88x92M) input values are set to zero. The demodulation of the OFDM signal is carried out by an FFT of length NFFT. The following parameters are also defined:
Txe2x80x94symbol duration used,
TGxe2x80x94guard interval, which is at least as long as the maximum channel echo, and
Fxcex94xe2x80x94subcarrier separation
The relationships TS=T+TG and Fxcex94=I/T apply. For practical applications, TG less than 0.25Ts.
A data block contains a sequence of at least one OFDM symbol gi(t). This is provided with a test signal, which is positioned either in front of the data block or in the middle of the data block. In the former case, the test signal is referred to as a preamble, and in the second case as a midamble. In a practical implementation of a multicarrier transmission system, it can be stated, as a precondition, that: the time characteristics of the transmission channel are approximately constant for the duration of the test signal 2TS; and the frequency characteristics of the transmission channel are approximately constant for a frequency interval of at least 2Fxcex94.
It is accordingly an object of the invention to provide a method and a configuration for a combined measurement of the start of a data block and a carrier-frequency shift in a multi-carrier transmission system for irregular transmission of data blocks that overcome the above-mentioned disadvantages of the prior art methods and devices of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for receiving a multicarrier signal, including a single transmission of a data block, in which case a cyclic multicarrier test signal having a cyclic time layout is transmitted together with the data block, and the cyclic multicarrier test signal being used for a combined measurement of a start of a first data symbol of the data block and of any carrier-frequency shift between a transmitter and a digital receiver, the cyclic multicarrier test signal being evaluated in the digital receiver performing the steps which includes:
performing a coarse measurement of the start of the data block by evaluating the cyclic multicarrier test signal without any fast fourier transformation (FFT) being calculated in the step;
determining a fine carrier-frequency shift, which may be present between the transmitter and the digital receiver, by isolation and evaluation of a part of the cyclic multicarrier test signal without any FFT being calculated in the step;
performing isolation and digital frequency correction of NFFT sample values from the cyclic multicarrier test signal and calculation of an FFT of length NFFT from the sample values resulting in calculated FFT values;
performing isolation and phase correction of a test vector of length M less than NFFT from the calculated FFT values resulting in a phase-corrected test vector;
calculating a further test vector of length Mxe2x88x921 by differential decoding of the phase-corrected test vector;
measuring any integer carrier-frequency shift which may be present between the transmitter and the digital receiver with a maximum value of xcex8 subcarrier intervals, in terms of magnitude, by isolation of at least L+2xcex8 values from the further test vector and by carrying out 2xcex8+1 correlations using a basic, known training sequence of length L;
performing a fine measurement of the start of the data block at least by correlation of the further test vector with a basic, known training sequence of maximum length Mxe2x88x921, which contains the known training sequence as a subsequence;
deriving an exact determination of the start of the first data symbol in the data block by addition of the start of the data block from the coarse measurement and a corrected value for the start of the data block from the fine measurement; and deriving an exact determination of a total carrier-frequency shift between the transmitter and the digital receiver by adding the integer carrier-frequency shift and the fine carrier-frequency shift.
The object of the invention is to specify a method which, controlled by a test signal which is transmitted once and taking into account minimum processing complexity, defines in a combined manner the precise start of the first data symbol and the carrier-frequency shift between the transmitter and receiver in a data block provided with the test signal.
According to the invention, the object is achieved by the receiver by monitoring the received signal and by the totality and sequence of method steps which specify the OFDM symbol clock for demodulation of the individual subcarrier symbols, drive digital frequency correction, and correct frequency synthesis in the radio-frequency section of the receiver.
An essential feature of the solution is that the multicarrier test signal having a cyclic time layout is transmitted with the data block. The multicarrier test signal is than evaluated in the receiver by a series of evaluation steps. It is then advantageous to transmit the single-carrier test signal before the multicarrier test signal, in order to use the single-carrier test signal to determine the start of the data block, additionally and with little processing complexity.
An essential feature of the solution is the optimum sequence relating all the method steps, in which the presence of parameters that are not yet known does not prevent the estimation of a parameter (time or frequency) in the respective step. Suitable isolation of test signals and test vectors results in no noise being caused by intersymbol interference (ISI) and subcarrier interference in the parameter estimation process.
A major advantage of the invention is the combined estimation of the symbol start and the frequency shift by a single synchronization sequence. The small number of computation operations for the synchronization sequence is advantageous, particularly the fact that only a single FFT is required for synchronization.
Another advantage of the invention is that, in the case of differential modulation on each individual subcarrier in the time domain, the FFT required for synchronization can be used, by use of subsequent phase correction for each usable subcarrier, to calculate the first OFDM reference symbol in the data block. Furthermore, after the phase correction, a channel estimate can be made in the frequency domain, which is used for coherent demodulation or for equalization of the subcarrier symbols in the frequency domain.
A major advantage of the invention for wire-free applications is the use of the single OFDM subcarrier, at times, for transmission of the single-carrier test signal. By narrowband filtering out of this test signal in a suitable manner and single-carrier operations, the receiver can determine the symbol clock and the start of the data block while saving power and with a small number of computation operations per unit time. This step is optional, and is particularly advantageous for start synchronization for mobile terminals, where time is not critical. It is assumed in this case that the OFDM module can be scaled appropriately for the single-carrier operating mode.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method and a configuration for a combined measurement of the start of a data block and a carrier-frequency shift in a multicarrier transmission system for irregular transmission of data blocks, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.